The present invention relates to a converging position detecting apparatus, in particular, of a type which detects a deviation of a converging position on a scanning surface of a light beam that is emitted from a light source and scans the scanning surface in a converged state.
In conventional scanning optical apparatuses, a laser beam modulated in accordance with an image signal is deflected by a deflector to scan a scanning surface (e.g., the surface of a photoreceptor body) while being converged into a spot image by a converging lens system, to thereby effect recording. These apparatuses are commonly used in, for instance, laser printers. In these apparatuses, the laser beam converging state on the photoreceptor body is detected by a photoelectric conversion element disposed at an end of a scanning line.
In the above scanning optical apparatuses, environmental changes such as a temperature change may cause such problems as thermal deformation in individual parts of the lens system, light source device, etc., to deviate the converging position of the laser beam perpendicularly to the scanning surface. As a result, the beam diameter is increased to lower the image quality.
For example, Japanese Patent Application Unexamined Publication Nos. Hei. 1-237614 and Hei. 4-155304 disclose techniques to solve the above problem.
The publication 1-237614 discloses a technique shown in FIG. 7. A laser beam L.sub.1 emitted from a laser light source unit 1 is converted to a parallel laser beam L.sub.2 by a collimating lens 2. The laser beam L.sub.2 is deflected by a rotary polygon mirror 3 and then converged by an f.theta. lens 5 to become a laser beam L.sub.3. The laser beam L.sub.3 impinges on two photodetecting means 11 and 12 that are spaced from a scanning surface perpendicularly in opposite directions. Each of the photodetecting means 11 and 12 consists of a photodetecting element and a slit which is narrower than a beam diameter. The converging position of the laser beam L.sub.3 is detected based on the beam diameters as detected by the photodetecting means 11 and 12, and corrected so as to be located on the scanning surface.
The publication 4-155304 discloses an apparatus having a pair of opposed knife edges that are arranged in a scanning direction of a laser beam with a gap wider than a beam diameter and spaced from the scanning surface in opposite directions. The apparatus further has a photodetecting element that is disposed in rear of the pair of knife edges and is wider than their gap, and a detecting circuit for detecting a deviation of a converging position of the laser beam based on factors including signal waveforms corresponding to the respective knife edges.
However, the above conventional techniques have the following problems.
In the scanning optical apparatus shown in FIG. 7, the converging apparatus is equipped with the two photodetecting means 11 and 12 each consisting of the photodetecting element and the slit narrower than the beam diameter, and is therefore costly. Further, differences in characteristics between the two photodetecting means 11 and 12 need to be corrected.
In the second conventional apparatus, a beam of a single scan is detected with the pair of knife edges and the one photodetecting element. Since the converging position is detected based on a rise and a fall of a beam, it is difficult to obtain accurate detection results because of a difference between rise and fall characteristics of the photodetecting element and their variations.